Implement proper update of urn:f0:type

[big-kahuna-burger]

1. Changing from m2m/web into native/spa => not allowed
2. Changing from native/spa into m2m/web => allowed, to be followed up with issuing a secret for a client and (maybe?) adding client_credentials to grant_types.

Token Endpoint Method is nonsecretive when token_endpoint_auth_method is one of (private_key_jwt, none, tls_client_auth, self_signed_tls_client_auth)
Corresponding default grant types: implicit (?), authorization_code, refresh_token

Token Endpoint Method is a secretive when token_endpoint_auth_method is not in above set.
Corresponding default grant types: implicit (?), authorization_code, refresh_token, client_credentials